The heatwave is at its worst in Europe. In the majority of European nations, temperatures have smashed all records. Persistently elevated temperatures can have negative impacts on the environment and human societies, including higher mortality rates, wildfires, and subpar food yields. With growing trends over the past 42 years that are three to four times faster than those in the other northern midlatitudes, we can say that Europe is a thermal hotspot. This rapid trend is linked to atmospheric dynamical changes by an increase in the occurrence and durability of dual jet stream states over Eurasia. We find that double jet occurrences are particularly important for heatwaves in Western Europe, where they can contribute up to 35% of temperature variation. The higher trend in the length of double jet events accounts for about all of the enhanced heatwave propensity in western Europe and roughly 30% of it across the extended European region. Since the findings demonstrate that atmospheric dynamical changes, in addition to thermodynamical causes, have been connected to the increased frequency of heatwaves in Europe, they are significant for risk management and future adaptation approaches.
Definition, statistics, and trends for heatwaves
Although there is no clear definition for what constitutes a heatwave, a variety of factors and benchmarks have been widely used and studied in the literature. A heatwave has three primary characteristics: physical, temporal, and geographical. By characterizing the temperature ranges reached, physical attributes can describe the intensity of a heatwave. The duration of a heatwave is hinted at by temporal characteristics. Here, a heatwave day is determined by the following criteria:
- Temperature threshold: based on a centered 15-day time frame, Fischer and Schär state that the daily maximum temperature must be higher than the top quintile of the highest temperature distribution of the study period (Tmax >90th percentile).
- Increasing temperatures are defined as occurring for at least 362 or 661 consecutive days when the temperature is above the threshold. The findings for shorter heatwaves (3 consecutive days) are published in the SI, but the results for longer heatwaves (6 consecutive days) are given in the main text.
- Geographic scope: We classify an occurrence as a heatwave if it exceeds an area of 40.000 km2 within a sliding window of 4° to 4°. The testing of various sliding windows revealed no discernible difference in heatwave detection.
We are worried about heat wave force in addition to heat wave frequency. The term “heatwave cumulative intensity,” according to Perkins-Kirkpatrick and Lewis3, refers to the integration of heat variance exceeding the threshold for each heatwave occurrence. When referring to a whole region, we additionally add up the heat exceedance throughout the entire geographical range:
Heatwave cumulative intensity=∑gp1∑d1(Tmax−Tmax90th)
where d is the number of consecutive heatwave days, gp is the number of land grid points for each area, Tmax is the highest daily temperature, and Tmax90th is the 90th percentile of the maximum temperature distribution for the entire period 1979–2020.
Because it incorporates the intensity, length, and spatial extent of a heatwave into one number, the heatwave cumulative intensity is a useful metric. Comparing different regions or years is made easier in this way. When the additional heat experienced after the wave threshold is crossed is taken into consideration, this parameter becomes more impact-relevant3.
In all cases where heatwave accumulated intensity is aggregated for a specific place, only the land grid points are taken into consideration, and the values are adjusted by the cosine of the latitude to account for the difference in grid cell sizes between different latitudinal zones.
Extreme heat has been on the rise globally in recent decades, and as global warming intensifies, this tendency is expected to continue. Europe has seen a particularly sharp rise in heat extremes since the devastating heatwave of the summer of 2003, which is likely to have caused an additional 70,000 deaths. This pattern is best illustrated by the recent run of back-to-back extremely hot, dry summers in 2018, 2019, and 2020. Although the underlying causes are not fully understood, it is expected that European heatwaves will grow disproportionately about the global mean temperature in the future.
The hot summers in Europe and the variability of heatwaves are influenced by large-scale convection, jet stream states, soil moisture shortage, related land-atmosphere feedbacks, oceanic circulation, and sea-surface temperatures. Anthropogenic global warming, which is mostly brought on by increasing GHGs, directly affects the intensity and frequency of heatwaves, but it can also affect these natural variability elements.
According to observational and model-based research, blocking anticyclones are mostly responsible for hot weather surges across northern midlatitudes. A twofold jet stream pattern across Eurasia, which is connected to weak winds between two zonal wind maxima, favors the formation of those obstructive high-pressure systems. Rossby wave-breaking and subsequent blocking may also cause the jet stream to split into several jets. In any event, the presence of a twin jet in the troposphere is indicated by a somewhat limited subtropical jet that can affect the Rossby wave in the midlatitudes and favor the stalling of ridges and troughs. Through the development of the polar jet front, high-latitude land warming during hotter months, which has been linked to human-caused climate change, may create ideal conditions for the occurrence or persistence of double jet states. Rossby wave equation also predicts that under strong Arctic Amplification, multiple jet flow regimes may become slightly more frequent as the zonal flow diminishes.
There is very little evidence that past or future global warming would change the severity and frequency of summer European blocks, despite the trend for heatwaves in Europe to increase. Modeling studies, on the other hand, have shown an atypical summertime high-pressure response that favors hot weather across western Europe and is located off the coast of the UK.
Why it is happening?
Global warming contributes to heat waves all around the world since temperatures now are typically roughly 1.1 degrees Celsius higher than they were in the late 19th century when the production of carbon dioxide and other gases became popular. As a result, extreme heat starts higher up. However, there are other factors, some involving air, and water flow, that could make Europe a hot zone for heat waves.
Do all circumstances have the same set of governing variables?
The concept of a normal heat wave doesn’t exist. On Monday, England and Wales experienced oppressive heat as a result of a belt of upper deck low-pressure air that had been trapped off the coast of Portugal for days. It is known as a “cutoff low” in the lingo of atmospheric scientists because it was cut off from the mid-latitude jet stream, a river of westerly winds that circles the planet at tremendous altitudes.
How about the heatwave that is affecting the rest of the continent?
Areas of low pressure are where air prefers to flow. In this case, the low-pressure region has been gradually attracting air from North Africa towards it and into Europe. It is pushing hot air northward, said Kai Kornhuber, a researcher at Columbia University’s Lamont-Doherty Earth Observatory.
In a study that was published last month, Kornhuber took part. It showed that during the four decades, extreme heat in Europe has increased in frequency and severity. The study also made a connection between this increase with changes in the jet stream, at least in part. The jet stream split into two, leaving a zone of feeble breezes and highly compressed air between the two branches, which is conducive to the buildup of extreme heat, as the researchers discovered that many heat waves in
Europe happened at this time.
Does ocean warming have any impact?
There may be other reasons for the increasing and protracted occurrence of heat waves in Europe, even though some of these are the subject of expert debate. According to Rousi, the fluctuation of the natural climate can make it difficult to distinguish particular effects.
The warming of the Arctic, which is occurring far more swiftly than in other parts of the world, could be a cause, according to Kornhuber. As it heats up faster, the temperature gap between the Arctic and the equator gets less. Due to the decrease in summertime winds, weather systems are prolonged as a result. We do see more tenacity, he continued. There are also suggestions that Europe’s temperature may be influenced by the Atlantic Meridional Overturning Circulation, one of the world’s strongest ocean currents. Using computer simulations, Rousi showed how a slowdown of the circulation as the world warmed would change air circulation, leading to drier summers in Europe in a study he published last year.
Double jet variability and trend in the European heatwave
Double jets have a significant impact on the reported increased European heatwave trend since they can account for up to 35% of the variability in heatwaves over parts of western Europe. Using linear regression analysis (see Methods), we show that double jet persistence explains a sizable percentage of the variability in the cumulative intensity of the European heatwave (above figure). To account for potential biases coming from the natural cycle and long-term trends, we first volume reduces both the regression model (double jet persistence) and the output variables (heatwave aggregate intensity) before doing the regression analysis. The fluctuation in the cumulative severity of heatwaves over areas of western Europe, including Spain and the Baltic states, is shown in Fig. a and can be attributed to double jet endurance up to 35 percent of the time. Regressing on heatwave frequency or double jet frequency (instead of endurance, see Fig. S6) yields similar, but less significant, results (instead of cumulative intensity, see Figs. S7, S8). To better understand the relationship between both jets and collected heat in Europe, we plot the double jet persistence against the regionally aggregated accumulated heat over Europe (Fig. b) and western Europe (Fig. c).
In both cases, as also seen in Fig. a, we find positive linear relationships that, when concentrated on western Europe, become statistically significant and considerably more obvious. In the European domain, the average explained variance is 5%, whereas in, western Europe, it is 24%. (Fig. c). The significance of sustained double jets for heatwaves in western Europe is illustrated by these results, which corroborate the conclusions from the composite of heat extremes.
Twin jets are becoming more persistent, and this may account for up to one-third of the increased European heatwave trend and nearly all of the rapid rise over western Europe (Fig.). The linear regression model was used to forecast the cumulative intensity trend of the heatwave at each grid point only based on the persistence of the twin jet (see Methods). We assume that the direct thermodynamic contributions to heatwave trends are similar across the midlatitudes. This first-order approximation provides an estimate of the thermodynamic contribution with a mean midlatitude trend of 0.62 °C/dec. The thermodynamic contribution is then subtracted from the overall trend to determine the “residual trend” for Europe, which is, on average, 1.05 °C/dec for the entire European region and 0.54 °C/dec for western Europe (Fig.). Thus, the augmented trend across Europe is the residual trend when contrasted to all mislaid latitudes we infer that this residual trend is caused by more complicated processes, such as feedback or dynamics. The decadal trend patterns over significant portions of Europe are fairly well captured by our linear model, but with lesser magnitudes (Fig. b, note difference in scale compared to a). This is predicted given that other variable such as soil moisture-temperature feedbacks15, undoubtedly influence or aggravate heatwave trends. However, the persistence of twin jet states, particularly over western Europe and European Russia, is increasingly capturing the residual tendency. Using this method, we conclude that, given the aforementioned assumptions. The increase in double jet endurance accounts for almost 30% of the entire study and knowledge in HW aggregate intensity over Europe. Even more drastically, the contribution from western Europe is significantly bigger and accounts for the whole residual trend. It is over 100%. This supports the hypothesis that dynamical trends toward more persistent double jets can account for the increasing trend of heatwaves in Western Europe.
A heat wave in Europe can enhance the likelihood of others occurring in the same area because, like in other parts of the world, a period of severe heat dries out the soil.
The water evaporates while the soil is moist, using part of the sun’s energy in the process, which has a very small cooling effect. There won’t be much moisture in the soil left for the second hot air wave to evaporate if one heat wave fully evaporates it. The temperature so increases as more sun energy bakes the surface.